I've noticed that:

1) After connecting monitor to the docking station's DP output following
outputs appear: DisplayPort-3, DisplayPort-4 and DisplayPort-5.
2) After disconnecting monitor from docking stating I can see:
[drm] DM_MST: stopping TM on aconnector: 00000000651bd996 [id: 57]
[drm] DM_MST: Disabling connector: 00000000e1b7686d [id: 90] [master:
00000000651bd996]
[drm] DM_MST: Disabling connector: 0000000078869ed2 [id: 94] [master:
00000000651bd996]
[drm] DM_MST: Disabling connector: 00000000af4e5879 [id: 98] [master:
00000000651bd996]
3) After disconnecting whole docking station I can still see DisplayPort-3,
DisplayPort-4 and DisplayPort-5 in the "xrandr" output.

Is that expected? Is that a possible hint for some problem with output
management that also causes the original problem?